In a cellular communication system, multiple antennas at a base station (BS) and multiple antennas at one or more user terminals (UTs) served by the BS allow two or more independent data streams to be transmitted from the BS to the UT(s) over the same time-frequency interval. The specific transmission technique that makes this possible is referred to as spatial multiplexing. In general, spatial multiplexing is a multiple-input, multiple-output (MIMO) transmission technique that uses the different “paths” or channels that exist between the multiple antennas at the BS and the multiple antennas at the one or more UTs to spatially multiplex the independent data streams over the same time-frequency interval. When one UT is served two or more independent data streams by the BS over the same time-frequency interval, the system is said to be performing single-user MIMO (SU-MIMO), and when multiple UTs are each served one or more independent data streams by the BS over the same time-frequency interval, the system is said to be performing multi-user MIMO (MU-MIMO).
The number of independent data streams that can be transmitted over the same time-frequency interval can be shown to be limited by the lesser of the number of antennas at the BS and the total number of antennas at the one or more UTs. A further limitation on the number of independent data streams that can be transmitted over the same time-frequency interval results from interference between the independent data streams or what is referred to as inter-user interference in the MU-MIMO context.
In T. L. Marzetta, “Noncooperative Cellular Wireless with Unlimited Numbers of Base Station Antennas,” IEEE Transactions on Wireless Communications, vol. 9, no. 11, pp. 3590-3600, November 2010 [Marzetta], concept referred to as “massive MIMO” was introduced. In general terms, massive MIMO refers to a communication system that has a large number of excess antennas available at the BS over the number of independent data streams to be transmitted over the same time-frequency interval. The excess antennas are used to reduce inter-user interference by further focusing the energy of each independent data stream into ever-narrower regions of space. This is done by appropriately shaping the independent data streams so that the wave fronts emitted by the available antennas for each of the independent data streams add up constructively at the location of the UT intended to receive the independent data stream and destructively everywhere else (or at least everywhere else where another UT is intended to receive a different independent data stream over the same time-frequency interval). The process of shaping the independent data streams at the BS is known as precoding.
Although inter-user interference can be reduced using the concept of massive MIMO, for a practical number of excess antennas at the BS, inter-user interference can still affect downlink data transmissions without proper scheduling.
The embodiments of the present disclosure will be described with reference to the accompanying drawings. The drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number.